1. Field of the invention
The present invention relates to a viscoelastic fluid. The present invention further relates to a method of fracturing a subterranean formation with a viscoelastic fluid.
2. Description of the Related Art
Viscoelastic surfactant (VES) fluids have continued to grow in use in oilfield applications because of their advantages over conventional polymer systems. Such advantages include higher permeability in the oil bearing zone, lower formation or subterranean damage, higher viscosifier recovery after fracturing, elimination of need for enzymes or oxidizers to break down viscosity, and easier hydration and faster build-up to optimum viscosity.
Growth in the use of VES fluids has been inhibited by the high cost of surfactants required to formulate such fluids. Another problem with use of VES fluids is their low tolerance of organic/inorganic salts and clay stabilizers, such as potassium chloride and tetramethyl ammonium chloride (TMAC), in subterranean formations. Another problem with use of VES fluids is the high temperatures encountered in deep well oilfield applications, i.e. up to 250° C. High temperatures can break down the viscosity of VES fluids and render them ineffective in fracturing operations when viscoelastic surfactants are present at low concentrations or require use of high viscoelastic surfactant concentrations to avoid such viscosity breakdown. Use of viscoelastic surfactants at low concentrations also can result in unacceptably long shear recovery time after high shear operation.
In the prior art, attempts have been made to remedy breakdown in viscosity performance by adding polymers and/or cosurfactants, such as low molecular weight anionic polymers. However, shear recovery can be unacceptably long and/or organic/inorganic salt tolerance may be inadequate.
Accordingly, it would be desirable to have a VES fluid that could be formulated on a cost-effective basis, i.e., with relatively low levels of viscoelastic surfactant. It would further be desirable to have a VES fluid that exhibits high tolerance with respect to organic/inorganic salts and clay stabilizers. It would still further be desirable to have a VES fluid with relatively low levels of viscoelastic surfactant that maintains a high level of viscosity performance at high temperatures and shear recovery comparable to fluids with a relatively high concentration of viscoelastic surfactants.